Abstract
Introduction
Femorotibial malalignment exceeding ±3° is a recognised contributor of early mechanical failure after total knee replacement (TKR). The angle between the mechanical and anatomical axes of the femur remains the best guide to restore alignment. We investigated where the femoral head lies relative to the pelvis and how its position varies with respect to recognised demographic and anatomic parameters. We have tested the hypothesis of the senior author that the position of the centre of the femoral head varies very little, and if its location can be identified, it could serve to outline the mechanical axis of the femur without the need for sophisticated imaging.
Patients & Methods
The anteroposterior standing, plain pelvic radiographs of 150 patients with unilateral total hip replacements were retrospectively reviewed. All patients had Tönnis grade 0 or 1 arthritis on the non-operated hip joint. All radiographs were obtained according to a standardised protocol. Using the known diameter of the prosthetic head for calibration, the perpendicular distance from the centre of the femoral head of the non-operated hip to the centre of pubic symphysis was measured with use of TraumaCad software. Anatomic parameters, including, but not limited to, the diameter of the intact femoral head, were also measured. Demographic data (gender, age, height, weight) were retrieved from our database.
Results
There were 72 men and 78 women with a mean age of 67.0 years. The mean distance of the centre of the femoral head to the pubic symphysis was 89.1 mm (standard deviation [SD], 5.8 mm). This distance was higher in men (mean, 90.5 mm; SD, 5.6 mm) than in women (mean, 87.7 mm; SD, 5.8 mm) (p=0.003). No statistically significant correlations between this distance and body-mass index or age (p=0.57 and p=0.28, respectively) were revealed. However, height was strongly correlated with this distance (r=0.52, p < 0.01), as was the diameter of the femoral head (r=0.61, p < 0.01). The latter was significantly larger in men than in women (p < 0.01).
Conclusion
This is the first study to demonstrate that the position of the centre of the femoral head has very little variability, irrespective of patient age or body-mass index. As expected, it varies with patient's height and gender. Although statistically significant, the difference between genders is < 3 mm and can be explained by the smaller femoral head size of females. Our findings should prove useful during TKR surgery, as they facilitate intra-operative identification of the centre of the femoral head. Based on these results, simple trigonometry shows that if a surgeon measures 9 cm from the symphysis pubis, a line from this point to the centre of the knee will be within 2° and 2.2° of the true mechanical axis in 98% (i.e. ± 3 SD of the mean) of male and female patients, respectively.
